In general, hygiene paper products are known comprising a functional as well as a decorative structure. In particular, these hygiene paper products are printed and then embossed. In an additional step, the hygiene paper product, which is typically made in a continuous form, is cut to discrete lengths as desired so as to form a single sheet or perforations are provided to constitute a line of weakness that enables the consumer to separate a single sheet from a plurality of sheets which may be present in the form of a roll, e.g. a toilet roll or a kitchen roll. Between the treatments, namely printing, embossing and cutting/perforating there is generally, no synchronization. That is, the functional or decorative modifications of a continuous web during converting depend on certain repeat rates, which are generally predetermined by the equipment used and, thus, not the same.
For example, the repeat rate for the printing decoration may be 378 mm, i.e., the printing decor length is 378 mm and is repeated every 378 mm. However, the embossing pattern, for example, is repeated every 30 mm. Consequently, the position of the print to the embossing is different in every single sheet, as the repeat rates of these treatments do not match. The same occurs if the perforation repeat length (sheet length) is also different, that is in the above example 250 mm. Thus, the position of the print and the embossing is different on every sheet, as the perforation repeat length also does not match the other repeat lengths. This leads, as shown in FIG. 1, to print designs disturbed by embossing and perforations everywhere in the design. One sheet 1 of the hygiene paper product includes an embossed pattern 2 and printed pattern 3. Further, the sheet 1 is cut or perforated along a separation line 4. Because the repeat rates of the three elements embossing, printing and perforating do not match, e.g. the body of a printed animal interferes with the embossing and is partly cut or intersected, respectively, by the separation line 4.
In order to improve the optical appearance of the product, EP-A-0958112 discloses to synchronize or register one of the functional structures or the decorative structure with cutting or perforating. In this context, the spatial relationship between one pattern and the lines of termination (i.e. the line that separates one sheet from another either by means of a perforation or line of weakness or cutting) are set by adjusting either the rate of applying the pattern or the rate of applying the perforation or cutting, wherein the sheet is transported at a constant velocity.
U.S. Pat. No. 3,594,552 discloses a system and method for synchronizing single printing modules with each other. All used printing cylinders are rotated in unison at the same peripheral speed and the repeat rates of each printing cylinder are the same. Any failure in the synchronization is detected by a scanner detecting a reference mark on the web to be printed and, additionally, by a rotary, digital encoder. If an error is detected, it is corrected by means of a compensating device.
Thus, to enable two patterns to be in register, a pattern size, i.e., the length in the direction of movement of the web (in machine direction), needs to fit the respective processes. In typical state of the art equipment using a two-roll-combination, the pattern length of printing or embossing is predetermined by the roll diameters, roll circumferences or an integer number of the circumferences, respectively. For example, a typical roll circumference of a printing press is 756 mm. Consequently, the pattern length predetermined by the roll circumference can be selected from 756, 378 or 252, etc., i.e. the circumference divided by an integer number will define the repeat lengths. The maximum achievable repeat length is 756. The same considerations apply to embossing, where an engraved steel embossing roll with a certain diameter defines a maximum repeat length and the feasible integer divisions. For example, an embossing roll diameter of 530 mm having a circumference of 1664 mm may be used.
In theory, the chosen repeat lengths of both printing and embossing have to be identical to get a matched printing/embossing decoration. With the aforementioned roll dimensions, a print repeat of 75.6 mm (10 repeats per revolution) would match and be in register, respectively, with an embossing repeat of 75.6 (22 repeats per revolution). However, such theory is only applicable if certain process parameters are neglected.
In particular, converting a material web requires a web tension greater than 0 in order to pull the web forward and to control web tracking. In addition, the printing process, as well as the embossing process, requires and creates web speed that matches roll surface speeds. That is, there is no slip. Further, the web elongation within the printing and embossing process varies. Moreover, state of the art control systems control web tension, so that the stress-strain relation of the substrate is varied. Especially in the case of a highly stretchable tissue, this results in varying elongation at constant web tension. Thus, a minimum error/difference in repeat lengths or web elongation or speed will add up over time. For example, after 1000 repeats of the above example with 75.6 mm, even a small difference of some micrometers will add up after less than two minutes. FIG. 2 schematically illustrates such a mismatch, which is also called “walking off pattern”.